0:00:32 > 0:00:36There are some seas where fish swarm in millions...
0:00:39 > 0:00:43..and plankton blossoms in vast clouds.
0:00:45 > 0:00:51The sheer quantity of life here is unmatched anywhere in the oceans.
0:00:58 > 0:01:02These are the most productive seas on Earth.
0:01:06 > 0:01:10They are the seasonal seas.
0:01:14 > 0:01:20These seas border the temperate regions where the seasons change.
0:01:27 > 0:01:32The seasons also affect the underwater world.
0:01:34 > 0:01:39The power of the sun is constantly changing.
0:01:40 > 0:01:46In the far north, during the summer, there are long hours of sunlight.
0:01:46 > 0:01:51In winter, that dwindles and there can be weeks of darkness.
0:01:52 > 0:01:55The summers are warm and gentle.
0:01:55 > 0:01:59The winters wracked by savage storms.
0:02:53 > 0:02:59In conditions like this, life of any sort has to struggle to survive.
0:03:04 > 0:03:08January on Sable Island, off Nova Scotia.
0:03:08 > 0:03:14Grey seals have got ashore through the crashing breakers.
0:03:21 > 0:03:25Gales here can blow for days on end.
0:03:44 > 0:03:49Sable Island has the world's largest colony of grey seals.
0:03:49 > 0:03:53100,000 come here to breed each year,
0:03:53 > 0:03:56just when the weather's at its worst.
0:04:01 > 0:04:04To add insult to injury,
0:04:04 > 0:04:09the pups - having suckled for only 18 days - are abandoned.
0:04:09 > 0:04:14Their mothers must return to the sea to find food for themselves.
0:04:17 > 0:04:24Unable to dive, the pups are marooned and sustained by nothing but their fatty blubber.
0:04:25 > 0:04:30It will be five weeks before they're strong enough to swim.
0:04:30 > 0:04:36By then, it will be early spring. The ocean will be teeming with food.
0:04:50 > 0:04:55By May, spring has reached the coasts of Scotland.
0:04:57 > 0:05:01Underwater, it arrived rather earlier.
0:05:01 > 0:05:07During March, the seas had warmed enough to trigger a transformation.
0:05:18 > 0:05:21These are phytoplankton -
0:05:21 > 0:05:25tiny floating algae, each much smaller than a pin head.
0:05:25 > 0:05:28They multiply with amazing speed
0:05:28 > 0:05:34to produce more annual growth than all the plants on land put together,
0:05:34 > 0:05:37six billion tonnes of it.
0:05:38 > 0:05:42This immense bloom spreads across the face of the ocean.
0:05:42 > 0:05:48Within a couple of months, it turns vast areas of it a dense green.
0:05:51 > 0:05:56Animal life reacts to the blooming sea.
0:05:56 > 0:06:00These polyps are about to change into something else.
0:06:00 > 0:06:06As they separate, they're revealed to be tiny common jellyfish.
0:06:10 > 0:06:13They're less than three millimetres across.
0:06:16 > 0:06:22Within a few months, they will have assembled into vast swarms.
0:06:26 > 0:06:30Minute copepods are part of their staple diet.
0:06:30 > 0:06:34These appear every spring in vast numbers
0:06:34 > 0:06:38and graze on the phytoplankton bloom.
0:06:40 > 0:06:43Their beating legs create currents
0:06:43 > 0:06:47that sweep the algae into the filters around their mouths.
0:06:52 > 0:06:55On this microscopic scale,
0:06:55 > 0:07:00water is so viscous, phytoplankton can't swim against the current.
0:07:01 > 0:07:06Whilst feeding, senses on the copepod's antennae
0:07:06 > 0:07:08warn of dangers ahead.
0:07:11 > 0:07:14Lighting by lasers
0:07:14 > 0:07:20reveals that feeding copepods leave wakes behind them, like jet trails.
0:07:38 > 0:07:41Slightly larger floating predators
0:07:41 > 0:07:46are able to use these trails to find their prey.
0:07:58 > 0:08:01A close call,
0:08:01 > 0:08:07but some predators are simply too large to avoid...
0:08:11 > 0:08:14..jellyfish.
0:08:17 > 0:08:20They may appear to be delicate
0:08:20 > 0:08:24but they are deadly hunters.
0:09:00 > 0:09:03With every pulse of the delicate bells,
0:09:03 > 0:09:08plankton-rich water is drawn into their lacey throats
0:09:08 > 0:09:14and pushed out again, leaving behind copepods stuck to the membranes.
0:09:21 > 0:09:26A single sea nettle jellyfish, only a few centimetres across,
0:09:26 > 0:09:30can collect thousands of copepods in a day.
0:09:41 > 0:09:47So the killing power of giants, like these, is hard to estimate.
0:09:51 > 0:09:55Each of these weighs up to 30 kilos
0:09:55 > 0:10:00and has tentacles stretching over eight metres.
0:10:03 > 0:10:08But there are even greater dangers awaiting the copepods.
0:10:08 > 0:10:13By the late spring, the baby common jellyfish are fully grown.
0:10:13 > 0:10:16They gather in millions,
0:10:16 > 0:10:19forming immense swarms
0:10:19 > 0:10:25which filter out all the small planktonic animals in their path.
0:11:10 > 0:11:16Nevertheless, there are such astronomic numbers of copepods
0:11:16 > 0:11:21that enough will survive to form swarms of their own.
0:11:24 > 0:11:27It's early spring in British Columbia.
0:11:27 > 0:11:31Seaweed has started to grow slowly in the cold water.
0:11:31 > 0:11:36As the hours of sunlight increase, and the water warms,
0:11:36 > 0:11:41these small plants turn into great beds of bull kelp.
0:11:52 > 0:11:56The immense 30-metre-long strands have small gas-filled floats,
0:11:56 > 0:12:00which keep them within reach of the energy-giving sunlight.
0:12:10 > 0:12:14Further south, the sunshine is more powerful.
0:12:14 > 0:12:20So here, on the coast of California, the biggest kelp can grow.
0:12:22 > 0:12:25This is giant kelp.
0:12:25 > 0:12:30By midsummer, each plant grows in length by a metre a day.
0:12:34 > 0:12:39Californian sea otters gather in the kelp forest
0:12:39 > 0:12:42to rest and snooze in safety.
0:12:44 > 0:12:50To prevent themselves being carried away into dangerous open water,
0:12:50 > 0:12:57where the big predators cruise, they anchor themselves by winding kelp around their body.
0:13:02 > 0:13:06Sooner or later, they have to find food.
0:13:06 > 0:13:10That lies on the sea bed, a long way below them.
0:13:10 > 0:13:14However, they can stay underwater for up to ten minutes.
0:13:14 > 0:13:17That's ample time to find shellfish.
0:13:48 > 0:13:55Many of the smaller creatures that live in these forests - such as urchins - graze on the kelp
0:13:55 > 0:13:59and can damage it if their numbers are unchecked.
0:13:59 > 0:14:05Sea otters feed on these grazers and prevent them getting too numerous.
0:14:05 > 0:14:09In effect, sea otters are the forest's guardians.
0:14:12 > 0:14:17Garibaldi fish do not, in fact, damage the kelp.
0:14:17 > 0:14:23They help it by picking off tiny animals that encrust the leaves.
0:14:28 > 0:14:33They graze on bryozoans - tiny colonial animals
0:14:33 > 0:14:39which build their colonies like a patchwork of white skins on leaves.
0:14:52 > 0:14:56When night falls, there are fewer predatory fish
0:14:56 > 0:15:01and the bryozoans emerge from their white shelters.
0:15:04 > 0:15:10Now, just like coral polyps, they start filtering out the plankton
0:15:10 > 0:15:12under cover of darkness.
0:15:16 > 0:15:21They're not the only animals to venture out at this time.
0:15:25 > 0:15:29This is an amphipod, just 2cm long.
0:15:29 > 0:15:32And it does eat kelp.
0:15:38 > 0:15:43In turn, it is excellent food for many predators.
0:15:48 > 0:15:52To protect itself, it produces silk, like a spider,
0:15:52 > 0:16:00and sews together two sides of a kelp frond and so form a shelter.
0:16:06 > 0:16:10This one is in particular need of a secure home.
0:16:10 > 0:16:13She's a mother.
0:16:16 > 0:16:20There are 50 youngsters clustered on her abdomen,
0:16:20 > 0:16:23so her home is becoming cramped.
0:16:25 > 0:16:29They will soon be old enough to leave
0:16:29 > 0:16:34and now, when she can, she kicks them out to get a taste of the world.
0:16:42 > 0:16:45Dawn.
0:16:45 > 0:16:51Beds of eel grass grow between the kelp forest and the shore.
0:16:52 > 0:16:56A harbour seal has found sanctuary here
0:16:56 > 0:17:01and is sleeping after a hard night's foraging.
0:17:02 > 0:17:04But not for long.
0:17:07 > 0:17:10A male seal gives a wake-up call.
0:17:15 > 0:17:20More seals are attracted from all directions.
0:17:26 > 0:17:31It's June - the time when young male harbour seals
0:17:31 > 0:17:34start their strange mating displays.
0:17:36 > 0:17:41One listens attentively to the grunting noises made by the other.
0:17:45 > 0:17:53These calls are almost certainly a way of establishing which of the two will be dominant.
0:17:56 > 0:18:01But if the animals are closely matched in size and experience,
0:18:01 > 0:18:04grunts won't settle the issue.
0:18:04 > 0:18:08The rivals will have to come to blows.
0:18:12 > 0:18:15Seals can be surprisingly violent.
0:18:28 > 0:18:32In a month's time, the breeding season will start,
0:18:32 > 0:18:35then fights will be in earnest.
0:18:35 > 0:18:39But now, in midsummer, these exchanges are harmless.
0:18:42 > 0:18:48Many of the creatures in the kelp have to venture out in order to feed.
0:18:48 > 0:18:51The bat ray, for one.
0:18:54 > 0:19:01Unlikely though it seems, the sandy floor of the open sea is, for the bat ray, a rich feeding ground.
0:19:14 > 0:19:18There is food, hidden within the sand.
0:19:18 > 0:19:22The bat ray has a special technique for finding it.
0:19:28 > 0:19:32It uses jets of water to blow the sand aside,
0:19:32 > 0:19:35and expose small invertebrates.
0:19:38 > 0:19:43A kelp bass hangs about alongside, waiting for scraps.
0:20:03 > 0:20:06Other hunters are also on the prowl.
0:20:06 > 0:20:09The fan-tailed sole.
0:20:22 > 0:20:26There are mantid shrimps here, living in tunnels.
0:20:29 > 0:20:34But, once again, hunger compels them into the open.
0:20:34 > 0:20:38That, of course, is a gamble.
0:20:39 > 0:20:42They will either eat,
0:20:42 > 0:20:44or be eaten.
0:20:52 > 0:20:56A sea slug called Janolus.
0:20:57 > 0:21:03Its colours suggest that it's poisonous, and so it is,
0:21:03 > 0:21:07to everything except another kind of sea slug...
0:21:10 > 0:21:13..the predatory Navanax.
0:21:17 > 0:21:22Navanax pulls itself along the trail of slime the Janolus leaves behind.
0:21:37 > 0:21:41Once caught, Janolus rolls into a ball.
0:21:46 > 0:21:51All Navanax gets, is a few yellow tentacles.
0:21:56 > 0:22:01And Janolus is swept to safety by the current.
0:22:07 > 0:22:09It's now midsummer.
0:22:09 > 0:22:14The sun is shining at full strength.
0:22:16 > 0:22:22The increasing warmth is the cue for an Atlantic lobster to start on a long journey.
0:22:22 > 0:22:27She's spent the winter 250m down, far beyond the reach of the storms.
0:22:29 > 0:22:34But it was cold down there, and now she needs to find warmer water,
0:22:34 > 0:22:38so she's marching towards the shallows.
0:22:38 > 0:22:42They, however, are 150km away.
0:22:47 > 0:22:53After a month of walking, she arrives at her favourite sand bank.
0:22:55 > 0:22:58But she's not the first here.
0:22:58 > 0:23:03Dozens of lobsters have already dug themselves homes in the sand,
0:23:03 > 0:23:07and they don't intend to surrender them to newcomers.
0:23:37 > 0:23:41Size counts for everything in these battles.
0:23:41 > 0:23:47The new arrival is in urgent need of a pit. Since she weighs seven kilos,
0:23:47 > 0:23:50she stands a good chance of getting one.
0:24:03 > 0:24:05She's won.
0:24:07 > 0:24:11These battles continue for the next two months.
0:24:11 > 0:24:16They're crucial, for the females need shelter and warm water,
0:24:16 > 0:24:19if they're to raise their young.
0:24:19 > 0:24:26For the last seven months, each of these females has been carrying around 20,000 fertilised eggs.
0:24:26 > 0:24:30But their task is approaching its end.
0:24:30 > 0:24:36The warmth of the shallows is speeding the eggs' development.
0:24:36 > 0:24:41Two more months, and the eggs are ready to hatch.
0:24:59 > 0:25:02At first, they're not very good at swimming.
0:25:04 > 0:25:09But within a few minutes, the babies can set off purposefully.
0:25:12 > 0:25:16At this time of year, the sea is full of larval animals.
0:25:16 > 0:25:20This one is a one-day-old lobster.
0:25:22 > 0:25:28And this, a three-week-old crab, ready to start life on the sea floor.
0:25:30 > 0:25:34Its feet touch the bottom for the first time.
0:25:34 > 0:25:41At this stage, it's a vegetarian with a taste for sea lettuce.
0:25:41 > 0:25:47As it grows, it will repeatedly moult and grow into a bigger, thicker skin.
0:25:47 > 0:25:49The chances are, it will be eaten.
0:25:49 > 0:25:56But if it survives for five years, it'll be a magnificent armoured giant.
0:26:02 > 0:26:05Now, it eats meat.
0:26:05 > 0:26:10Special adaptations enable it to hunt in the dark.
0:26:14 > 0:26:18Its jointed feet are covered in sensors,
0:26:18 > 0:26:24which detect the slightest chemical change in its surroundings.
0:26:24 > 0:26:29As it walks in the darkness, its feet can, literally, taste the sand.
0:26:31 > 0:26:37As soon as it finds suitable food, it passes it to its crushing claws,
0:26:37 > 0:26:40which make light work of the soft flesh.
0:26:40 > 0:26:44Those claws are also very useful for defence.
0:26:50 > 0:26:55A 1.5-metre-long common octopus glides by.
0:26:58 > 0:27:04The jet-propelled giant is both powerful and very clever.
0:27:10 > 0:27:15With octopus about, it's risky for even a crab to be in the open.
0:27:15 > 0:27:22Even in the dark, the octopus' eyes are sensitive to the slightest movement.
0:27:30 > 0:27:36Against a hunter like this, the crab's claws are useless.
0:27:45 > 0:27:48Late summer in south-east Alaska.
0:27:53 > 0:27:59The water is still warming and mysid shrimp swarm near the surface.
0:28:05 > 0:28:09It's a final feast for Pacific salmon
0:28:09 > 0:28:13returning to the coast from the open Pacific.
0:28:18 > 0:28:24They're heading inshore to breed and they arrive in huge numbers.
0:28:25 > 0:28:28They have to swim far up the rivers to spawn.
0:28:28 > 0:28:32But this river's level is too low.
0:28:32 > 0:28:37They'll have to wait until rain causes it to rise.
0:28:38 > 0:28:44They are trapped close to the sea shore - the worst place to be.
0:28:47 > 0:28:50A three-metre-long salmon shark,
0:28:50 > 0:28:53a close relation of the great white.
0:28:56 > 0:29:01This one's sensed minute electrical signals from the salmon nearby.
0:29:01 > 0:29:06Shark can maintain their blood temperature
0:29:06 > 0:29:11at a higher level than the surrounding sea water.
0:29:11 > 0:29:13That means they can be quick.
0:29:19 > 0:29:22Quicker even than salmon.
0:29:41 > 0:29:46Early autumn in Vancouver Island in Canada, 600 miles to the south.
0:29:47 > 0:29:51The ocean temperature is slowly dropping.
0:29:51 > 0:29:5840 metres below the surface, baby herring feed on the last of the summer's plankton.
0:29:58 > 0:30:03Their movements attract attention from the skies above.
0:30:10 > 0:30:13Gulls can't dive,
0:30:13 > 0:30:17so for now the fish are still safe.
0:30:19 > 0:30:22But there are birds which can dive.
0:30:25 > 0:30:30Auklets and murres swim effortlessly down beneath the school.
0:30:30 > 0:30:35The panicked herring are forced towards the surface.
0:30:43 > 0:30:49They gather into a giant defensive ball of swirling fish.
0:30:50 > 0:30:54The commotion attracts yellow-tailed rockfish.
0:30:55 > 0:30:58They too are hunters.
0:31:07 > 0:31:10The marauding fish scatter the herring.
0:31:19 > 0:31:24Repeated attacks split the ball into numerous smaller groups.
0:31:29 > 0:31:32Now it's easier for the divers
0:31:32 > 0:31:37to keep the confused fish penned at the surface.
0:31:37 > 0:31:40There, even gulls can get at them.
0:31:56 > 0:32:02Attacked from all sides, the fish have virtually no chance.
0:32:03 > 0:32:08More and more divers are attracted to the scene.
0:32:10 > 0:32:13They harry the shrinking numbers of herring
0:32:13 > 0:32:18right down to the very last individual.
0:32:33 > 0:32:36Bigger predators cruise here, too.
0:32:37 > 0:32:41Pacific white-sided dolphin.
0:32:43 > 0:32:47The dolphin are mainly nocturnal hunters.
0:32:47 > 0:32:50During the day, they socialise.
0:32:50 > 0:32:55They display by releasing streams of bubbles
0:32:55 > 0:32:58and they play games.
0:33:00 > 0:33:04Games like pass-the-seaweed, for example.
0:33:37 > 0:33:42Exactly eight months ago, off the west coast of Scotland,
0:33:42 > 0:33:47an egg was laid and securely fixed to a strand of kelp.
0:33:47 > 0:33:51Inside, a tiny embryo started to develop.
0:33:59 > 0:34:02Protected by the tough egg case,
0:34:02 > 0:34:06it endured the worst of the winter storms.
0:34:14 > 0:34:17By summmer, it was half-grown.
0:34:28 > 0:34:31Now, at last, in the late autumn
0:34:31 > 0:34:35it's nearly ready to hatch.
0:34:42 > 0:34:46A fully-formed miniature shark swims free.
0:34:46 > 0:34:53This year's plankton will soon die, but the young dogfish can hunt immediately for larger prey.
0:34:56 > 0:34:59The days are getting shorter now.
0:35:00 > 0:35:06In British Columbia, the water begins to chill.
0:35:06 > 0:35:11This bizarre-looking creature is searching the kelp for food.
0:35:11 > 0:35:15This is melibe - the hooded sea slug.
0:35:18 > 0:35:23It catches plankton with its net-like head.
0:35:31 > 0:35:36As winter approaches, plankton is becoming scarce.
0:35:36 > 0:35:40But melibe is an assiduous searcher.
0:35:42 > 0:35:45It can swim.
0:35:49 > 0:35:54It flaps away to look for a better feeding spot.
0:35:54 > 0:35:57But its search is becoming difficult.
0:35:57 > 0:36:02Within the next two weeks, most of the plankton will have disappeared.
0:36:11 > 0:36:16As the sunlight becomes feeble, the kelp starts to die,
0:36:16 > 0:36:19gradually rotting away to nothing.
0:36:50 > 0:36:53Soon it will be winter.
0:36:56 > 0:36:59But 9,000 miles to the south,
0:36:59 > 0:37:03the sun is rising on a new spring day.
0:37:04 > 0:37:09The southern hemisphere too has temperate regions.
0:37:09 > 0:37:13The plankton is beginning to bloom around Tasmania.
0:37:13 > 0:37:17Just as in the north,
0:37:17 > 0:37:22the southern seasonal seas have areas of rich green water,
0:37:22 > 0:37:25with their own kelp forests...
0:37:27 > 0:37:30..and their own swarms of plankton.
0:37:37 > 0:37:43Some of these inhabitants live only in the southern hemisphere.
0:37:43 > 0:37:49This is one of them. The handfish, that strolls around on modified fins.
0:37:53 > 0:37:59But when needs must, it can resort to tail power.
0:38:03 > 0:38:08Every summer, visitors come to the shallows around Tasmania.
0:38:08 > 0:38:14These are Australian squid, about half a metre long.
0:38:15 > 0:38:18They are here to breed.
0:38:22 > 0:38:27The larger males compete for the attentions of a female,
0:38:27 > 0:38:30displaying towards her
0:38:30 > 0:38:35by putting on a ballet, where they continually change their colour.
0:39:04 > 0:39:07Eventually, they form pairs.
0:39:13 > 0:39:19The male passes a package of sperm across to the female.
0:39:21 > 0:39:24After the eggs have been fertilised,
0:39:24 > 0:39:28they're deposited in tough, rubbery egg cases
0:39:28 > 0:39:31that other creatures find poisonous.
0:39:33 > 0:39:38Within three weeks, the babies are ready to hatch out.
0:39:43 > 0:39:47They are already able to change colour,
0:39:47 > 0:39:49but they're not such good swimmers.
0:40:06 > 0:40:11There's another animal here that is a rather more devoted parent.
0:40:12 > 0:40:15This is a male leafy sea dragon,
0:40:15 > 0:40:20an exquisitely-decorated relative of the sea horse.
0:40:21 > 0:40:26He is carrying his partner's eggs around with him.
0:40:26 > 0:40:31They'd be a nutritious snack for any predator that found them.
0:40:31 > 0:40:38That's not easy, because they're attached to his perfectly camouflaged body.
0:40:38 > 0:40:42They could scarcely be in a safer place.
0:41:05 > 0:41:08By November,
0:41:08 > 0:41:1210,000 miles to the north, winter has arrived.
0:41:12 > 0:41:17Norway gets under 5 hours of daylight in every 24.
0:41:17 > 0:41:21The temperature is falling rapidly.
0:41:22 > 0:41:27But despite the cold, the sea is far from deserted.
0:41:27 > 0:41:31Every winter, 500 million tonnes of adult herring
0:41:31 > 0:41:34seek shelter in these deep waters.
0:41:37 > 0:41:39They will stay here for four months,
0:41:39 > 0:41:45living on fat they accumulated during summer feasts of plankton.
0:41:45 > 0:41:48But they're not alone.
0:41:48 > 0:41:50Orca.
0:41:50 > 0:41:56This pod is part of a population of some 500 killer whales,
0:41:56 > 0:42:01that specialize in hunting North Atlantic herring.
0:42:02 > 0:42:05THEIR CRIES ECHO
0:42:12 > 0:42:18Using their echolocation, they've detected a shoal of herring
0:42:18 > 0:42:2150 metres below them.
0:42:21 > 0:42:27With enough air for a ten-minute dive, they swim below the herring
0:42:27 > 0:42:30and drive the fish upwards.
0:42:34 > 0:42:37Bubbles stream from the rising fish
0:42:37 > 0:42:43as gas in their swim bladders expands and escapes.
0:42:46 > 0:42:52Even an orca finds it difficult to catch a healthy herring.
0:42:52 > 0:42:57But they have a devastating weapon all their own.
0:43:03 > 0:43:09They use their tail to club the fish with waves of water pressure.
0:43:15 > 0:43:21Then, it's simply a matter of collecting the stunned casualties.
0:43:35 > 0:43:38The herrings have no chance
0:43:38 > 0:43:43and both orca and gulls will eat as much as they can
0:43:43 > 0:43:46every day for the next four months.
0:43:48 > 0:43:54But there are so many fish wintering here - over 5 billion individuals -
0:43:54 > 0:43:58that the losses are almost unnoticeable.
0:44:04 > 0:44:08Violent as this winter weather may be,
0:44:08 > 0:44:13it's essential for the renewal of the riches of the seasonal seas.
0:44:13 > 0:44:20Out in the open oceans, the surging waters stir up nutrients from the depths.
0:44:23 > 0:44:29By the end of winter, the seas will be full of minerals once more,
0:44:29 > 0:44:35ready for the return of the sun and the next great plankton bloom.
0:44:44 > 0:44:51Many creatures in the Blue Planet have never been filmed before and some are hardly known to science.
0:44:51 > 0:44:58One such is a relative of the infamous Great White Shark, the Alaskan Salmon Shark.
0:44:58 > 0:45:03Until recently, these sharks had only been encountered by local fishermen,
0:45:03 > 0:45:10but often it has been an alliance between film-makers and scientists that's brought new insights.
0:45:10 > 0:45:15To get these pictures, a Blue Planet crew joined with a research group,
0:45:15 > 0:45:19making one of the first ever studies of this extraordinary shark.
0:45:23 > 0:45:30Each summer, hundreds of Salmon Sharks visit Alaska's Prince William Sound
0:45:30 > 0:45:37in search of salmon returning to breed in freshwater rivers. Fishermen want to start harvesting the sharks
0:45:37 > 0:45:40and there is an urgent need for research.
0:45:40 > 0:45:46The shark's lifestyle is a mystery and no-one knows how big the population is.
0:45:46 > 0:45:53Biologist Ken Goldman wants to learn more, but first he has to catch his study animal.
0:45:53 > 0:45:55Lower, now!
0:45:59 > 0:46:01- Down!- OK.
0:46:01 > 0:46:03OK.
0:46:03 > 0:46:08They work as quickly as possible, to mimimise stress to the shark.
0:46:20 > 0:46:26Down, tag, 9-8-1-6-3. You got pre-caudal and feet, flip it over...
0:46:26 > 0:46:32Marking sharks with special tags will help reveal how many there are and where they go.
0:46:32 > 0:46:38- You want to cut?- No, pre-caudal, do it now! Sixty pre-caudal, fork! - Ninety.- Total?- A hundred.
0:46:38 > 0:46:42- Let's get this fish out of here. - Go ahead, start down.
0:46:42 > 0:46:48Catching and quickly releasing sharks four metres long is not easy work,
0:46:48 > 0:46:51but it's essential to the long-term survival of the species.
0:46:51 > 0:46:52All right.
0:46:54 > 0:47:00- One down. - One down, five hundred and...- As many more of those as we can get.
0:47:01 > 0:47:06Nearby our crew were trying to film the sharks' natural behaviour.
0:47:06 > 0:47:12They have a mini-camera on a pole and a viewing monitor on the surface,
0:47:12 > 0:47:17but to get reasonable pictures, the sharks have to come close.
0:47:17 > 0:47:24I think one of the things that people don't realise is just how long these sequences take to film.
0:47:24 > 0:47:31I've worked on this two years, setting it up, and Peter and I will be here four weeks.
0:47:31 > 0:47:39So, all we've seen is a few fins around and so we've been charging after these sharks, individual ones.
0:47:39 > 0:47:45All we've managed is to keep up behind them and just get some shots of the tail.
0:47:45 > 0:47:50We can film without getting in the water, using the pole cam system,
0:47:50 > 0:47:57which can produce very good results. But generally you get better results in the water.
0:47:57 > 0:47:59Just a slight safety issue!
0:47:59 > 0:48:05You don't get in the water unless you're bloody confident these things will be benign.
0:48:05 > 0:48:09Getting in with these wouldn't worry me.
0:48:09 > 0:48:13But we obviously must be very safety-conscious
0:48:13 > 0:48:19cos the last thing we'll do is get in if there's a risk of being eaten. Peter says they're fish-eaters
0:48:19 > 0:48:22and therefore don't pose a threat.
0:48:23 > 0:48:27My concern would be that we wouldn't get pictures.
0:48:27 > 0:48:34I certainly want to get in just to see them - they're superb creatures. But I'm slightly more nervous,
0:48:34 > 0:48:36cos after all, they're sharks!
0:48:36 > 0:48:43Curiosity did get the better of our team and they cautiously entered the water.
0:48:43 > 0:48:50With these sharks there is no prior art. It's the first time anybody's filmed them or even swum with them.
0:48:50 > 0:48:56It's certainly pretty scary and underwater visibility was ten or twelve feet.
0:48:56 > 0:49:03All you saw was a big curtain of green. A shark could come at you from underneath, behind, in front...
0:49:03 > 0:49:07and you'd have very little notice of it.
0:49:11 > 0:49:13- Lots of green.- Lots of green!
0:49:13 > 0:49:19As soon as we dropped in the water, they sank out of sight.
0:49:20 > 0:49:26It's only in the last ten years that Salmon Sharks have been sighted here in such large numbers.
0:49:26 > 0:49:30At times the crew saw 100 swimming near the surface -
0:49:30 > 0:49:35behaviour which might help these sharks regulate body temperature -
0:49:35 > 0:49:42but also means they're regularly caught in salmon fishermen's nets.
0:49:39 > 0:49:42OK, we got problems.
0:49:45 > 0:49:47INDISTINCT VOICES
0:49:47 > 0:49:54On this occasion it was for their own good, or at least for the conservation of the species,
0:49:54 > 0:50:01as Ken works with local fishermen to increase the number of tags on his study group.
0:50:03 > 0:50:09- NOISE OF WINCH - Okay, big girl. Everybody back, everybody just back.
0:50:09 > 0:50:14Only for a month each summer do sharks appear in Prince William Sound
0:50:14 > 0:50:19and otherwise nobody sees them or knows where they go.
0:50:19 > 0:50:21It's OK, sweetie, it's OK. It's OK.
0:50:23 > 0:50:28Over 90% of the animals Ken catches are females
0:50:28 > 0:50:35and he hopes that his tagging will help reveal the breeding grounds so that at least these can be protected.
0:50:37 > 0:50:42This is almost the only close up view that Ken gets of his sharks,
0:50:42 > 0:50:47so he was fascinated to see what our crew came up with.
0:50:58 > 0:51:03See you later, bye, girl. Good job, gentlemen.
0:51:03 > 0:51:06Way to go captain, beautiful.
0:51:10 > 0:51:16The crew were into their third week of filming. Time to compare notes with Ken.
0:51:16 > 0:51:18- That's nice.- Dink!
0:51:18 > 0:51:21- That's nice.- Right on the lens!
0:51:21 > 0:51:26This kind of activity is what I DON'T get to see - like that.
0:51:26 > 0:51:28That's splendid.
0:51:28 > 0:51:35Our vision under water is terrible - they're seeing that camera at 30 feet.
0:51:35 > 0:51:39- Well...- Their vision is adapted to that environment.
0:51:39 > 0:51:44When you look at them, they have a really pointy face.
0:51:44 > 0:51:50- Their eyes are very forward. - That explains why we can sneak up behind them.
0:51:50 > 0:51:54But once the camera sees their eye, bang - they're off.
0:51:54 > 0:51:58Ken hadn't seen his sharks so close before.
0:51:58 > 0:52:04To get those shots of the sharks coming at the lens, we had to put dead salmon
0:52:04 > 0:52:09that we'd got from the fishermen, close to the camera.
0:52:09 > 0:52:14The sharks would come and we'd get those close-up shots of the teeth and eyes.
0:52:14 > 0:52:19Just under the boat and he's gonna come up right under the bait.
0:52:21 > 0:52:27He's right at the bait, oh, he's dragging it. Oh-ho-ho-ho!
0:52:29 > 0:52:32LAUGHTER
0:52:38 > 0:52:43- Was that a shot? - Yes, it was a very good shot!
0:52:47 > 0:52:53- It was great, there were sharks everywhere.- We got some really nice shots today.
0:52:53 > 0:52:59It was amazing at one point, because wherever you looked, you saw shark fins.
0:52:59 > 0:53:02One or two of them actually charged straight at the lens.
0:53:31 > 0:53:34Subtitles by BBC Subtitling
0:53:35 > 0:53:38E-mail us at subtitling@bbc.co.uk